Data may be encoded using radiation by time modulating a radiation source. For example, in visible light communications, the intensity of light produced from a light source, such as an LED, may be modulated over time in order to encode data in a light signal. A photo-detector can then be used to receive the time-modulated signal which is decoded to reveal the data that was transmitted by the light source.
A digital camera can be conveniently used to receive the signal, which is then processed to extract the encoded data. In order to achieve an acceptably high transmission rate and achieve communication without obvious light flickering by the transmitting light source(s), the transmitting light source must be switchable between intensity levels at a suitably high rate. Conversely, the photo-detector must meet certain requirements, for example, by having an image capture rate fast enough to distinguish between the intensity transitions. However, at the same time, it would be beneficial if such communications methods could be used with common or off the shelf apparatus.
In certain communications methods, particularly in visible light communications, it is desirable to change the power of the radiation emitted by the transmitter, i.e. to use dimming of the radiation source. However, these changes can affect the data transmission capabilities.
Various techniques are employed in the art in an attempt to address this problem. For example, in the IEEE 802.15.7 standard, a method referred to as variable pulse position modulation (VPPM) is used. This involves changing the pulse duration depending on the dimming level required. In VPPN, the data rate is independent of the dimming level, but the bandwidth efficiency is poor. Other techniques employed to address this problem involve changes of the light intensity level. Problems associated with at least some of these techniques include degradation of the data rate performance for high dimming (low optical power).
Orthogonal frequency division multiplexing (OFDM) methods are popular for modulating signals in order to transmit data over dispersive channels. However, it is desirable to reduce the power consumption of communication systems. For example, this may be to maximise battery life for portable devices, or simply to save operating costs or reduce energy usage.
A variation on the OFDM modulation scheme, called SIM-OFDM, has been proposed in order to reduce the power required by communications devices relative to those that use traditional OFDM. The SIM-OFDM technique is described in “Subcarrier Index Modulation OFDM” by R. Abualhiga and H. Haas, in Proc. of the International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Tokyo, Japan, Sep. 13-16, 2009.
SIM-OFDM introduces an additional dimension alongside conventional OFDM encoding, the additional dimension coming from the state, i.e. active or inactive, of each frequency carrier available. In this way, frequency carrier states (i.e. used or unused) are used to encode data according to an on-off keying modulation scheme. As in OFDM, each active carrier transmits a signal that is modulated using a conventional modulation scheme such as but not limited to M-QAM. Each inactive carrier is set to a zero state. Hence, the power used to convey each M-QAM signal can also be used to encode further data by simply being present or not in a particular frequency carrier band. The SIM-OFDM concept is illustrated in FIG. 1.
In this case, the incoming bit stream is divided into blocks of bits, each having a length of N(0.5*log 2(M)+1), where N is the number of frequency carriers, and M is the constellation size of the respective M-QAM modulation scheme that is used. Each of these blocks is divided into two parts. The first N bits of the block form a first sub-block (BOOK). The remaining 0.5*N.log2(M) bits form a second sub-block (BQAM). The first sub-block (BOOK) is inspected and the majority bit type is determined by checking which bit value, 1 or 0, has most occurrences. The frequency carriers that have the same position inside the OFDM frame as the bits from the majority bit type in BOOK are classified as “active”, and the rest of the frequency carriers (i.e. those that correspond to the minority bit type) are classified as “inactive”. Inactive carriers are given the amplitude value 0+0j, where j=√−1. The first 0.5*N active frequency carriers are given amplitude values corresponding to the M-QAM constellation symbols necessary to encode the second sub-block (BQAM). The remaining active carriers can be used to signal the majority bit type of BOOK to the destination receiver and they will be assigned a signal whose power is equal to the average power for the given M-QAM scheme. Afterwards, an N-point IFFT transformation is performed in order to obtain the time-domain signal, which is transmitted.
In this way, for example, if the binary sequence [0 1 0 0 0 1 1 1 0 1 0 1] is to be transmitted using 4-QAM and 6 carriers, then the sequence is divided into a first sub-block [1 1 0 1 0 1] and a second sub-block [0 1 0 0 0 1]. The second sub-block is modulated into frequency carriers using 4-QAM modulation. Since the majority bit in the first sub-block is 1, then an active carrier is chosen to represent 1. In this case, the 4-QAM modulated signals are transmitted on the first, third and fifth frequency carrier channels. The sixth carrier, which is also active, can be used to convey to the destination what the majority bit type in BOOK is. It will be allocated power equal to the average power of the respective M-QAM scheme. Its positive amplitude will represent the majority bit type—in this case 1. This carrier channel allocation effectively encodes the first sub-block as [1 1 0 1 0 1].
A slight modification of SIM-OFDM involves signalling the majority bit type either through secure communication channels, or by reserving one particular frequency carrier and transmitting the desired value with a sufficiently high signal to noise ratio. It should also be noted that this modulation scheme saves power from all inactive carriers at the expense of spectral efficiency. The described configuration has been referred to as Power Saving Policy (PSP). In an alternative embodiment, for each single OFDM frame, the unused power from the inactive carriers can be reallocated to the active ones, which could lead to a performance enhancement.
Once a signal has been received by the receiver at the destination, it is transformed into the frequency domain with a fast Fourier transform operation. Then all the frequency carriers are inspected. Those carriers whose power is above a predetermined threshold are marked as active, and the rest of the carriers are marked as inactive. At least half of the total number of carriers are active. Hence, in case that less than 0.5*N active carriers are detected, the threshold value is decreased by a small step and the inspection is performed again. This procedure is done iteratively until at least 0.5*N active carriers are detected. Then the first sub-block (BOOK) is reconstructed from the detected states of the carriers and the known majority bit type. Afterwards, the first 0.5*N active carriers are demodulated according to the respective M-QAM scheme in order to reconstruct the second sub-block (BQAM) in the conventional manner. The spectral efficiency of this scheme is:
                    log        2            ⁡              (        M        )              2    +      1    ⁢          bits      carrier      
It is an object of at least one embodiment of the present invention to improve the performance of the SIM-OFDM scheme. The bit error rate (BER) performance of SIM-OFDM in an Additive White Gaussian Noise (AWGN) channel is illustrated in FIG. 2.
It is at least one object of at least one embodiment of the present invention to provide an improved or alternative communication system and detector and/or to at least partially address at least one problem with the prior art.